Mechanistic and computational studies of oxidatively-induced aryl-CF3 bond-formation at Pd: rational design of room temperature aryl trifluoromethylation

Anti-selective Cyclopropanation of Nonconjugated Alkenes with Diverse Pronucleophiles via Directed Nucleopalladation

A facile approach to obtaining densely functionalized cyclopropanes is described. The reaction proceeds under mild conditions via the directed nucleopalladation of nonconjugated alkenes with readily available pronucleophiles and gives excellent yields and good anti-selectivity using I2 and TBHP as oxidants. Pronucleophiles bearing a diverse collection of electron-withdrawing groups, including -CN, -CO2R, -COR, -SO2Ph, -CONHR, and -NO2, are well tolerated. Internal alkenes, which are generally challenging substrates in other cyclopropanation methods, provide excellent yields and good diastereoselectivity in this methodology, allowing for controlled access to cyclopropanes substituted at all three C atoms. DFT calculations and mechanistic experiments reveal that the major mechanistic pathway involves the initial α-iodination of the nucleophile, followed by anti-carbopalladation and intramolecular C(sp3)-I oxidative addition. Strain-release-promoted C(sp3)-C(sp3) reductive elimination then furnishes the cyclopropanated product.

Direct arene trifluoromethylation enabled by promiscuous activity of fungal laccase

Laccase from Trametes versicolor was found to oxidize non-phenolic arenes and enable the trifluoromethylation of arenes in the presence of in situ generated CF3 radicals at a catalyst loading as low as 0.0034%. The biocatalytic trifluoromethylation proceeded under mild conditions and could increase the yield by up to 12 fold, compared to the control.

Cross-Coupling through Ag(I)/Ag(III) Redox Manifold

In ample variety of transformations, the presence of silver as an additive or co-catalyst is believed to be innocuous for the efficiency of the operating metal catalyst. Even though Ag additives are required often as coupling partners, oxidants or halide scavengers, its role as a catalytically competent species is widely neglected in cross-coupling reactions. Most likely, this is due to the erroneously assumed incapacity of Ag to undergo 2e^- redox steps. Definite proof is herein provided for the required elementary steps to accomplish the oxidative trifluoromethylation of arenes through Ag^I /Ag^III redox catalysis (i. e. CEL coupling), namely: i) easy Ag^I /Ag^III 2e^- oxidation mediated by air; ii) bpy/phen ligation to Ag^III ; iii) boron-to-Ag^III aryl transfer; and iv) ulterior reductive elimination of benzotrifluorides from an [aryl-Ag^III -CF₃ ] fragment. More precisely, an ultimate entry and full characterization of organosilver(III) compounds [K]⁺ [Ag^III (CF₃ )₄ ]^- (K-1), [(bpy)Ag^III (CF₃ )₃ ] (2) and [(phen)Ag^III (CF₃ )₃ ] (3), is described. The utility of 3 in cross-coupling has been showcased unambiguously, and a large variety of arylboron compounds was trifluoromethylated via [Ag^III (aryl)(CF₃ )₃ ]^- intermediates. This work breaks with old stereotypes and misconceptions regarding the inability of Ag to undergo cross-coupling by itself.

Synthesis of Well-Defined High-Valent Palladium Complexes by Oxidation of Their Palladium(II) Precursors

The last decade has witnessed the rapid development of high-valent Pd-involved organic transformations. This has also led to a steadily growing number of publications concerning the preparation of isolable and characterizable palladium(III) and palladium(IV) complexes. A variety of one-electron and two-electron oxidants have been employed to give access to high-oxidation-state Pd compounds. Undoubtedly, the study of these stoichiometric reactions has great implications for relevant Pd-mediated catalysis. In this minireview, the focus is on the synthetic approaches to structurally determined Pd^III/IV complexes starting from their Pd^II precursors, and the advances in this research area from early 2010 to late 2019 will be highlighted. Chemical oxidations exploiting various oxidizing agents including 1) hypervalent iodine reagents; 2) halogens; 3) electrophilic fluorination reagents; 4) alkyl/aryl halides; 5) ferrocenium salts; 6) peroxides/O₂ ; 7) sulfonyl chlorides; and 8) others are covered. A "greener" electrooxidation manner has also been reviewed.

Aryl Sulfonium Salts for Site-Selective Late-Stage Trifluoromethylation

Incorporation of the CF₃ group into arenes has found increasing importance in drug discovery. Herein, we report the first photoredox-catalyzed cross-coupling of aryl thianthrenium salts with a copper-based trifluoromethyl reagent, which enables a site-selective late-stage trifluoromethylation of arenes. The reaction proceeds with broad functional group tolerance, even for complex small molecules on gram scale. The method was further extended to produce pentafluoroethylated derivatives.

Palladium-Catalyzed Asymmetric Allylic Fluoroalkylation/Trifluoromethylation

The first palladium-catalyzed asymmetric allylic trifluoromethylation is disclosed. The methodology evokes a fundamental principle by which the synergistic interplay of a leaving group and its subsequent activation of the nucleophilic trifluoromethyl group enabled the reaction. Allyl fluorides have been shown to be superior precursors for generation of π-allyl complexes, which lead to trifluoromethylated products with high selectivities and functional group tolerance. This study highlights the unique role of a bidentate diamidophosphite ligand class in palladium-catalyzed reactions that allow a challenging transformation to proceed.

Three-component difluoroalkylation–thiolation of alkenes by iron-facilitated visible-light photoredox catalysis

The first difluoroalkylation–thiolation of alkenes catalyzed by iron-facilitated photoredox has been developed with a broad substrate scope under mild conditions.

Copper Promoted Regio- and Stereoselective Aminochlorination of Alkynes and Alkenes with NFSI

A simple and rapid copper-promoted aminochlorination of unactivated alkynes and alkenes with N-fluorobenzenesulfonimide (NFSI) was developed. Two series of chloroenamines and chloroamines were obtained in good to high yields. The chlorinated enamines could be obtained in a single E configuration. This reaction involved a radical process and the CuCl₂ acted as the Cl source and NFSI as the N source.

DFT studies on the distinct mechanisms of C-H activation and oxidation reactions mediated by mononuclear- and binuclear-palladium

A series of density functional theory calculations have been carried out to investigate the detailed mechanisms of C-H activation and oxidation reactions, and further to disclose the distinct effects of mononuclear- and binuclear-palladium on these reaction pathways. The results of calculations demonstrated that the C-H activation of 2-phenylpyridine with mononuclear Pd(OAc)2 prefers the inner-shell proton-abstraction mechanism, while that with binuclear Pd2(μ-OAc)4 is biased to the outer-shell proton-abstraction mechanism. The rate-determining free-energy barriers of the two mechanisms were calculated to be 24.2 and 24.8 kcal mol-1, respectively. More importantly, we have simulated the oxidation pathways of PdII → PdIII and PdII → PdIV with strong oxidants including PhI(OAc)2, PhICl2 and NCS, and found that a binuclear PdII-precursor would be oxidized to the corresponding binuclear PdIII-complex while a mononuclear PdII-precursor was deemed to evolve to the corresponding mononuclear PdIV-complex. In addition, the oxidation of PdII with PhI(OAc)2 has been characterized as a radical mechanism, in sharp contrast to the ion-pair mechanism prevalent for the oxidation of PdII with PhICl2. The calculated kinetic and thermodynamic parameters could be qualitatively consistent with the related experimental observations. This molecular modelling can provide valuable insights into the understanding of the distinct effects of the resting state and oxidant on these important transformations.

Palladium-Catalyzed Decarbonylative Trifluoromethylation of Acid Fluorides

While acid fluorides can readily be made from widely available or biomass-feedstock-derived carboxylic acids, their use as functional groups in metal-catalyzed cross-coupling reactions is rare. This report presents the first demonstration of Pd-catalyzed decarbonylative functionalization of acid fluorides to yield trifluoromethyl arenes (ArCF3 ). The strategy relies on a Pd/Xantphos catalytic system and the supply of fluoride for transmetalation through intramolecular redistribution to the the Pd center. This strategy eliminated the need for exogenous and detrimental fluoride additives and allows Xantphos to be used in catalytic trifluoromethylations for the first time. Our experimental and computational mechanistic data support a sequence in which transmetalation by R3 SiCF3 occurs prior to decarbonylation.

Three-component difluoroalkylation and trifluoromethylthiolation/trifluoromethylselenolation of π-bonds

This report describes a new method for three-component difluoroalkylation and trifluoromethylthiolation/trifluoromethylselenolation of π-bonds via air-stable SCF₃ and SeCF₃ reagents as free-radical initiators of ethyl iododifluoroacetate. β-Proton elimination can be overcome effectively in this reaction system, and a broad substrate scope, including alkenes and alkynes, makes this approach practical and attractive.

Synthesis, Characterization, and Reactivity of Palladium Fluoroenolate Complexes

Cross-coupling reactions of aryl groups with α-fluoro carbonyl compounds catalyzed by palladium complexes have been reported, but palladium fluoroenolate intermediates relevant to such reactions have not been isolated or even detected previously. We report the synthesis, structural characterization, and reactivity of a series of C-bound arylpalladium fluoroenolate complexes ligated by monophosphines and bisphosphines. DPPF-ligated arylpalladium fluoroenolate complexes (DPPF = 1,1-bis(diphenylphosphino)-ferrocene) derived from a monofluoroester, a difluoroester, difluoroamides, and difluoroacetonitrile underwent reductive elimination in high yields. Reductive elimination was faster from complexes containing less electron-withdrawing fluoroenolate groups and longer Pd-C(enolate) bonds than from complexes containing more electron-withdrawing fluoroenolate groups and shorter Pd-C(enolate) bonds. The rates of reductive elimination from these C-bound fluoroenolate complexes were significantly faster than those of the analogous trifluoromethyl complexes.

C-H Bond Trifluoromethylation of Arenes Enabled by a Robust, High-Valent Nickel(IV) Complex

The robust, high-valent Ni^IV complex [(Py)₂ Ni^IV F₂ (CF₃ )₂ ] (Py=pyridine) was synthesized and fully characterized by NMR spectroscopy, X-ray diffraction, and elemental analysis. It reacts with aromatic compounds at 25 °C to form the corresponding benzotrifluorides in nearly quantitative yield. The monomeric and dimeric Ni^III CF₃ complexes 2⋅Py and 2 were identified as key intermediates, and their structures were unambiguously determined by EPR spectroscopy and X-ray diffraction. Preliminary kinetic studies in combination with the isolation of reaction intermediates confirmed that the C-H bond-breaking/C-CF₃ bond-forming sequence can occur both at Ni^IV CF₃ and Ni^III CF₃ centers.

Trifluoromethylation of a Well-Defined Square-Planar Aryl-NiII Complex involving NiIII /CF3. and NiIV -CF3 Intermediate Species

Ni-mediated trifluoromethylation of an aryl-Br bond in model macrocyclic ligands (L_n -Br) has been thoroughly studied, starting with an oxidative addition at Ni⁰ to obtain well-defined aryl-Ni^II -Br complexes ([L_n -Ni^II ]Br). Abstraction of the halide with AgX (X=OTf^- or ClO₄^- ) thereafter provides [L_n -Ni^II ](OTf). The nitrate analogue has been obtained through a direct C-H activation of an aryl-H bond using Ni^II salts, and this route has been studied by X-ray absorption spectroscopy (XAS). Crystallographic XRD and XAS characterization has shown a tight macrocyclic coordination in the aryl-Ni^II complex, which may hamper direct reaction with nucleophiles. On the contrary, enhanced reactivity is observed with oxidants, and the reaction of [L_n -Ni^II ](OTf) with CF₃⁺ sources afforded L_n -CF₃ products in quantitative yield. A combined experimental and theoretical mechanistic study provides new insights into the operative mechanism for this transformation. Computational analysis indicates the occurrence of an initial single electron transfer (SET) to 5-(trifluoromethyl)dibenzothiophenium triflate (TDTT), producing a transient L₁ -Ni^III /CF₃^. adduct, which rapidly recombines to form a [L₁ -Ni^IV -CF₃ ](X)₂ intermediate species. A final facile reductive elimination affords L₁ -CF₃ . The well-defined square-planar model system studied here permits to gain fundamental knowledge on the rich redox chemistry of nickel, which is sought to facilitate the development of new Ni-based trifluoromethylation methodologies.

Catalytic Intermolecular Carboamination of Unactivated Alkenes via Directed Aminopalladation

An intermolecular 1,2-carboamination of unactivated alkenes proceeding via a Pd(II)/Pd(IV) catalytic cycle has been developed. To realize this transformation, a cleavable bidentate directing group is used to control the regioselectivity of aminopalladation and stabilize the resulting organopalladium(II) intermediate, such that oxidative addition to a carbon electrophile outcompetes potential β-hydride elimination. Under the optimized reaction conditions, a broad range of nitrogen nucleophiles and carbon electrophiles are compatible coupling partners in this reaction, affording moderate to high yields. The products of this reaction can be easily converted to free γ-amino acids and γ-lactams, both of which are common structural motifs found in drug molecules and bioactive compounds. Reaction kinetics and DFT calculations shed light on the mechanism of the reaction and explain empirically observed reactivity trends.

Hypervalent Iodine Reagents in High Valent Transition Metal Chemistry

Over the last 20 years, high valent metal complexes have evolved from mere curiosities to being at the forefront of modern catalytic method development. This approach has enabled transformations complimentary to those possible via traditional manifolds, most prominently carbon-heteroatom bond formation. Key to the advancement of this chemistry has been the identification of oxidants that are capable of accessing these high oxidation state complexes. The oxidant has to be both powerful enough to achieve the desired oxidation as well as provide heteroatom ligands for transfer to the metal center; these heteroatoms are often subsequently transferred to the substrate via reductive elimination. Herein we will review the central role that hypervalent iodine reagents have played in this aspect, providing an ideal balance of versatile reactivity, heteroatom ligands, and mild reaction conditions. Furthermore, these reagents are environmentally benign, non-toxic, and relatively inexpensive compared to other inorganic oxidants. We will cover advancements in both catalysis and high valent complex isolation with a key focus on the subtle effects that oxidant choice can have on reaction outcome, as well as limitations of current reagents.

Copper-catalyzed regioselective 1,2-thioamidation of alkenes

An efficient method for regioselective 1,2-thioamidation of terminal alkenes, catalyzed by copper in the presence of NFSI and thiols, has been disclosed.

The first nucleophilic C–H perfluoroalkylation of aromatic compounds via (arene)tricarbonylchromium complexes

Complexed and reversibly activated arenes selectively react with a perfluoroalkyl anion under mild conditions, forming cyclohexadienyl adducts that readily undergo oxidation to perfluoroalkyl arenes.

Stability and Unimolecular Reactivity of Palladate(II) Complexes [Ln PdR3 ]- (L=Phosphine, R=Organyl, n=0 and 1)

The reduction of Pd^II precatalysts to catalytically active Pd⁰ species is a key step in many palladium-mediated cross-coupling reactions. Besides phosphines, the stoichiometrically used organometallic reagents can afford this reduction, but do so in a poorly understood way. To elucidate the mechanism of this reaction, we have treated solutions of Pd(OAc)₂ and a phosphine ligand L in tetrahydrofuran with RMgCl (R=Ph, Bn, Bu) as well as other organometallic reagents. Analysis of these model systems by electrospray- ionization mass spectrometry found palladate(II) complexes [L_n PdR₃ ]^- (n=0 and 1), thus pointing to the occurrence of transmetallation reactions. Upon gas-phase fragmentation, the [L_n PdR₃ ]^- anions preferentially underwent a reductive elimination to yield Pd⁰ species. The sequence of the transmetallation and reductive elimination, thus, constitutes a feasible mechanism for the reduction of the Pd(OAc)₂ precatalyst. Other species of interest observed include the Pd^IV complex [PdBn₅ ]^- , which did not fragment via a reductive elimination but lost BnH instead.

Biaryl Phosphine Based Pd(II) Amido Complexes: The Effect of Ligand Structure on Reductive Elimination

Kinetic studies conducted under both catalytic and stoichiometric conditions were employed to investigate the reductive elimination of RuPhos (2-dicyclohexylphosphino-2',6'-diisopropoxybiphenyl) based palladium amido complexes. These complexes were found to be the resting state in Pd-catalyzed cross-coupling reactions for a range of aryl halides and diarylamines. Hammett plots demonstrated that Pd(II) amido complexes derived from electron-deficient aryl halides or electron-rich diarylamines undergo faster rates of reductive elimination. A Hammett study employing SPhos (2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl) and analogues of SPhos demonstrated that electron donation of the "lower" aryl group is key to the stability of the amido complex with respect to reductive elimination. The rate of reductive elimination of an amido complex based on a BrettPhos-RuPhos hybrid ligand (2-(dicyclohexylphosphino)-3,6-dimethoxy-2',6'-diisopropoxybiphenyl) demonstrated that the presence of the 3-methoxy substituent on the "upper" ring of the ligand slows the rate of reductive elimination. These studies indicate that reductive elimination occurs readily for more nucleophilic amines such as N-alkyl anilines, N,N-dialkyl amines, and primary aliphatic amines using this class of ligands.